"The doors, inside and outside, and a panic-alarm that unlocks them sound like a great idea."
Almost, although many schools DO have that type of entrance/exit at the school's main entrance(because of THIS guy, back in 2001.
"Mamoru Takuma (宅間 守[a], Takuma Mamoru, 23 November 1963 – 14 September 2004) was a Japanese mass murderer who killed eight children in the Osaka school massacre in Ikeda, Osaka Prefecture, on 8 June 2001.[1]
Takuma had a long history of mentally disturbed and anti-social behavior, and an extensive criminal record including a conviction for rape.[2] He stabbed to death eight students and seriously wounded fifteen others at Ikeda Elementary School in a knife attack that lasted several minutes.
Takuma was convicted in August 2003 and executed on 14 September 2004."
(And, if I remember correctly, 7 of the 8 murdered were all 1st grade girls.)
The schools that do have the entrance like that are locked 24/7, and have either a guard there to let people in, or the 'parents' must push a speaker/camera button.
The Panic button in each classroom will automatically alert the police (which will definitely be there in less than 4 minutes) and set of a very loud alarm in each and every room at the school, where the teachers in the classrooms, or gym, will evacuate all of the students (usually to the playground (like we did for fire/tornado/nuke drills as kids)). There will be designated staff (usually not teachers, whom have to take care of the kids) to get what is needed to hand to the teachers to protect the kids (including the pole I posted a picture of). Teachers also go through a training session to learn to disarm a knife wielder.
Anyway, here is looking toward the back of a typical elementary school class (kids look like 1st (most likely, because their names are still taped to their desks) or 2nd grade):
(and from the looks of the class, that is 1 unhappy teacher, and a roomful of guilty kids that just got yelled at 😉)
Notice the doors in the back of the class.
The set on the left are going into the hallway (there is another matching set directly to the left of the picture taker, you can almost see the doors).
The doors to the right are the ones going outside, and when they are not in use, they are usually locked.
The panic button would be almost right behind the camera person, on the wall, along with the pole in the corner on the left (also behind the camera person).
Apologies for the confusion (it is kind of hard to explain without visual references).. 😝